Liquid-crystalline medium

ABSTRACT

Disclosed are a liquid-crystalline medium, which contains one or more compounds of formula I  
                 
their use for electro-optical purposes, to displays containing this medium, and to novel LC compounds for use in LC media and displays.

The present invention relates to a liquid-crystalline medium (LCmedium), to the use thereof for electro-optical purposes, to LC displayscontaining this medium, and to novel LC compounds for use in the LCmedia and displays.

Liquid crystals are used principally as dielectrics in display devices,since the optical properties of such substances can be modified by anapplied voltage. Electro-optical devices based on liquid crystals areextremely well known to the person skilled in the art and can be basedon various effects. Examples of such devices are cells having dynamicscattering, DAP (deformation of aligned phases) cells, guest/host cells,TN cells having a twisted nematic structure, STN (supertwisted nematic)cells, SBE (superbirefringence effect) cells and OMI (optical modeinterference) cells. The commonest display devices are based on theSchadt-Helfrich effect and have a twisted nematic structure.

The liquid-crystal materials must have good chemical and thermalstability and good stability to electric fields and electromagneticradiation. Furthermore, the liquid-crystal materials should have lowviscosity and produce short addressing times, low threshold voltages andhigh contrast in the cells.

They should furthermore have a suitable mesophase, for example a nematicor cholesteric mesophase for the above-mentioned cells, at the usualoperating temperatures, i.e. in the broadest possible range above andbelow room temperature. Since liquid crystals are generally used asmixtures of a plurality of components, it is important that thecomponents are readily miscible with one another. Further properties,such as the electrical conductivity, the dielectric anisotropy and theoptical anisotropy, have to satisfy various requirements depending onthe cell type and area of application. For example, materials for cellshaving a twisted nematic structure should have positive dielectricanisotropy and low electrical conductivity.

For example, for matrix liquid-crystal displays with integratednon-linear elements for switching individual pixels (MLC displays),media having large positive dielectric anisotropy, broad nematic phases,relatively low birefringence, very high specific resistance, good UV andtemperature stability and low vapour pressure are desired.

Matrix liquid-crystal displays of this type are known. Examples ofnon-linear elements which can be used to individually switch theindividual pixels are active elements (i.e. transistors). The term“active matrix” is then used, where a distinction can be made betweentwo types:

-   -   1. MOS (metal oxide semiconductor) or other diodes on silicon        wafers as substrate.    -   2. Thin-film transistors (TFTs) on a glass plate as substrate.

The use of single-crystal silicon as substrate material restricts thedisplay size, since even modular assembly of various part-displaysresults in problems at the joints.

In the case of the more promising type 2, which is preferred, theelectro-optical effect used is usually the TN effect. A distinction ismade between two technologies: TFTs comprising compound semiconductors,such as, for example, CdSe, or TFTs based on polycrystalline oramorphous silicon. Intensive work is being carried out worldwide on thelatter technology.

The TFT matrix is applied to the inside of one glass plate of thedisplay, while the other glass plate carries the transparentcounterelectrode on its inside. Compared with the size of the pixelelectrode, the TFT is very small and has virtually no adverse effect onthe image. This technology can also be extended to fully colour-capabledisplays, in which a mosaic of red, green and blue filters is arrangedin such a way that a filter element is opposite each switchable pixel.

The TFT displays usually operate as TN cells with crossed polarisers intransmission and are backlit.

The term MLC displays here encompasses any matrix display withintegrated non-linear elements, i.e., besides the active matrix, alsodisplays with passive elements, such as varistors or diodes(MIM=metal-insulator-metal).

MLC displays of this type are particularly suitable for TV applications(for example pocket televisions) or for high-information displays forcomputer applications (laptops) and in automobile or aircraftconstruction. Besides problems regarding the angle dependence of thecontrast and the response times, difficulties also arise in MLC displaysdue to insufficiently high specific resistance of the liquid-crystalmixtures [TOGASHI, S., SEKIGUCHI, K., TANABE, H., YAMAMOTO, E.,SORIMACHI, K., TAJIMA, E., WATANABE, H., SHIMIZU, H., Proc. Eurodisplay84, September 1984: A 210-288 Matrix LCD Controlled by Double StageDiode Rings, p. 141 ff, Paris; STROMER, M., Proc. Eurodisplay 84,September 1984: Design of Thin Film Transistors for Matrix Addressing ofTelevision Liquid Crystal Displays, p. 145 ff, Paris]. With decreasingresistance, the contrast of an MLC display deteriorates, and the problemof after-image elimination may occur. Since the specific resistance ofthe liquid-crystal mixture generally drops over the life of an MLCdisplay owing to interaction with the interior surfaces of the display,a high (initial) resistance is very important in order to obtainacceptable lifetimes. In particular in the case of low-volt mixtures, itwas hitherto impossible to achieve very high specific resistance values.It is furthermore important that the specific resistance exhibits thesmallest possible increase with increasing temperature and after heatingand/or UV exposure. The low-temperature properties of the mixtures fromthe prior art are also particularly disadvantageous. It is demanded thatno crystallisation and/or smectic phases occur, even at lowtemperatures, and the temperature dependence of the viscosity is as lowas possible. The MLC displays from the prior art thus do not satisfytoday's requirements.

Besides liquid-crystal displays which use backlighting, i.e. areoperated transmissively and if desired transflectively, reflectiveliquid-crystal displays are also particularly interesting. Thesereflective liquid-crystal displays use the ambient light for informationdisplay. They thus consume significantly less energy than backlitliquid-crystal displays having a corresponding size and resolution.Since the TN effect is characterised by very good contrast, reflectivedisplays of this type can even be read well in bright ambientconditions. This is already known of simple reflective TN displays, asused, for example, in watches and pocket calculators. However, theprinciple can also be applied to high-quality, higher-resolution activematrix-addressed displays, such as, for example, TFT displays. Here, asalready in the trans-missive TFT-TN displays which are generallyconventional, the use of liquid crystals of low birefringence (Δn) isnecessary in order to achieve low optical retardation (d·Δn). This lowoptical retardation results in usually acceptably low viewing-angledependence of the contrast (cf. DE 30 22 818). In reflective displays,the use of liquid crystals of low birefringence is even more importantthan in transmissive displays since the effective layer thicknessthrough which the light passes is approximately twice as large inreflective displays as in transmissive displays having the same layerthickness.

Thus, there continues to be a great demand for MLC displays having veryhigh specific resistance at the same time as a large working-temperaturerange, short response times, even at low temperatures, and a lowthreshold voltage which do not exhibit these disadvantages or only do soto a lesser extent.

In the case of TN (Schadt-Helfrich) cells, media are desired whichfacilitate the following advantages in the cells:

-   -   extended nematic phase range (in particular down to low        temperatures)    -   switchability at extremely low temperatures (outdoor use,        automobiles, avionics)    -   increased resistance to UV radiation (longer life)    -   low threshold voltage.

The media available from the prior art do not enable these advantages tobe achieved while simultaneously retaining the other parameters.

In the case of supertwisted (STN) cells, media are desired whichfacilitate greater multiplexability and/or lower threshold voltagesand/or broader nematic phase ranges (in particular at low temperatures).To this end, a further widening of the available parameter latitude(clearing point, smectic-nematic transition or melting point, viscosity,dielectric parameters, elastic parameters) is urgently desired.

In particular in the case of LC displays in equipment for mobileapplications (for example mobile telephones, PDAs), a significantreduction in the operating voltage is desired in order to reduce thetotal energy requirement of the equipment. To this end, a significantincrease in the polarity or the dielectric anisotropy of the LC media isnecessary. At the same time, a lowering of the clearing point and thespecific resistance and an excessive increase in the birefringence inthe LC media should be avoided. However, it has been found that the useof LC compounds of high polarity in LC media frequently results in alowering of the clearing point and/or an increase in the birefringenceas well as a reduction in the stability of the operating voltage onexposure to light and on heating.

The invention has the object of providing media, in particular for MLC,TN or STN displays of this type, which do not exhibit the disadvantagesmentioned above or only do so to a lesser extent, and preferably have avery low threshold voltage at the same time as high dielectricanisotropy, a high clearing point, large specific resistance and lowbirefringence.

It has now been found that this object can be achieved if LC mediacomprising one or more compounds of the formula I are used. Thecompounds of the formula I result in mixtures having the desiredproperties indicated above.

The invention relates to a liquid-crystalline medium (LC medium),characterised in that it comprises one or more compounds of the formulaI

in which

-   -   R⁰ denotes a halogenated or unsubstituted alkyl or alkoxy        radical having 1 to 15 C atoms, where, in addition, one or more        CH₂ groups in these radicals may each, independently of one        another, be replaced by    -    in such a way that O atoms are not linked directly to one        another,    -   X⁰ denotes F, Cl, CN, SF₅, SCN, NCS, a halogenated alkyl        radical, a halogenated alkenyl radical, a halogenated alkoxy        radical or a halogenated alkenyloxy radical having up to 6 C        atoms, and    -   Y^(1,2) each, independently of one another, denote H or F, and        each, independently of one another, denote        where at least one of the rings        denotes

The invention furthermore relates to the use of LC media as describedabove and below for electro-optical purposes, in particular in LCdisplays, preferably in MLC, TN and STN displays.

The invention furthermore relates to an electro-optical LC display, inparticular an MLC, TN or STN display, containing an LC medium asdescribed above and below.

The invention also relates to novel compounds of the formula I and tothe use thereof in LC media and LC displays as described above andbelow.

The compounds of the formula I have a relatively high clearing point,high positive dielectric anisotropy, low birefringence and a broadnematic phase range. Surprisingly, it has been found that LC mediacomprising compounds of the formula I simultaneously have high polarity,a very low threshold voltage and a high clearing point.

The LC media according to invention furthermore have, in particular, thefollowing advantages:

-   -   high “voltage holding ratio” (HR) after heating and/or exposure        to light,    -   low to very low birefringence, depending on the desired        application,    -   a broad nematic phase range,        and are therefore particularly suitable for LC displays in        mobile applications.

The compounds of the formula I have a broad range of applications.Depending on the choice of substituents, they can serve as basematerials of which liquid-crystalline media are predominantly composed;however, liquid-crystalline base materials from other classes ofcompound can also be added to the compounds of the formula I in order,for example, to modify the dielectric and/or optical anisotropy of adielectric of this type and/or to optimise its threshold voltage and/orits viscosity.

In the pure state, the compounds of the formula I are colourless andform liquid-crystalline mesophases in a temperature range which isfavourably located for electro-optical use. They are stable chemically,thermally and to light.

The compounds of the formula I are prepared by methods known per se, asdescribed in the literature (for example in the standard works, such asHouben-Weyl, Methoden der organischen Chemie [Methods of OrganicChemistry], Georg-Thieme-Verlag, Stuttgart), to be precise underreaction conditions which are known and suitable for the said reactions.Use can also be made here of variants known per se, which are notmentioned here in greater detail. The compounds of the formula I canalso be prepared by the processes described in WO 2004/048501 A1 and WO2006/125511 A1 or analogously thereto.

If R⁰ in the formulae above and below denotes an alkyl radical and/or analkoxy radical, this may be straight-chain or branched. It is preferablystraight-chain, has 2, 3, 4, 5, 6 or 7 C atoms and accordinglypreferably denotes ethyl, propyl, butyl, pentyl, hexyl, heptyl, ethoxy,propoxy, butoxy, pentoxy, hexyloxy or heptyloxy, furthermore methyl,octyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl,methoxy, octyloxy, nonyloxy, decyloxy, undecyloxy, dodecyloxy,tridecyloxy or tetradecyloxy.

Oxaalkyl preferably denotes straight-chain 2-oxapropyl (=methoxymethyl),2-(=ethoxymethyl) or 3-oxabutyl (=2-methoxyethyl), 2-, 3- or4-oxapentyl, 2-, 3-, 4- or 5-oxahexyl, 2-, 3-, 4-, 5- or 6-oxaheptyl,2-, 3-, 4-, 5-, 6- or 7-oxaoctyl, 2-, 3-, 4-, 5-, 6-, 7- or 8-oxanonyl,2-, 3-, 4-, 5-, 6-, 7-, 8- or 9-oxadecyl.

If R⁰ denotes an alkyl radical in which one CH₂ group has been replacedby —CH═CH—, this may be straight-chain or branched. It is preferablystraight-chain and has 2 to 10 C atoms. Accordingly, it denotes, inparticular, vinyl, prop-1- or -2-enyl, but-1-, -2- or -3-enyl, pent-1-,-2-, -3- or -4-enyl, hex-1-, -2-, -3-, -4- or -5-enyl, hept-1-, -2-,-3-, -4-, -5- or -6-enyl, oct-1-, -2-, -3-, -4-, -5-, -6- or -7-enyl,non-1-, -2-, -3-, -4-, -5-, -6-, -7- or -8-enyl, dec-1-, -2-, -3-, -4-,-5-, -6-, -7-, -8- or -9-enyl.

If R⁰ denotes an alkyl or alkenyl radical which is at leastmonosubstituted by halogen, this radical is preferably straight-chain,and halogen is preferably F or Cl. In the case of polysubstitution,halogen is preferably F. The resultant radicals also includeperfluorinated radicals. In the case of monosubstitution, the fluorineor chlorine substituent may be in any desired position, but ispreferably in the ω-position.

In the formulae above and below, X⁰ is preferably F, Cl or mono- orpolyfluorinated alkyl or alkoxy having 1, 2 or 3 C atoms or mono- orpolyfluorinated alkenyl having 2 or 3 C atoms. X⁰ is particularlypreferably F, Cl, CF₃, CHF₂, OCF₃, OCHF₂, OCFHCF₃, OCFHCHF₂, OCFHCHF₂,OCF₂CH₃, OCF₂CHF₂, OCF₂CHF₂, OCF₂CF₂CHF₂, OCF₂CF₂CHF₂, OCFHCF₂CF₃,OCFHCF₂CHF₂, OCF₂CF₂CF₃, OCF₂CF₂CClF₂, OCClFCF₂CF₃ or CH═CF₂, veryparticularly preferably F or OCF₃.

Particularly preferred compounds of the formula I are selected from thefollowing sub-formulae:

in which Y¹, Y², R⁰ and X⁰ have the meaning indicated in formula I. R⁰preferably denotes straight-chain alkyl having 1 to 8 C atoms,furthermore alkenyl having 2 to 7 C atoms.

Particular preferences given to compounds of the formulae Ia, Ib and Ic.

Particularly preferred compounds of the formulae I and Ia-Id are thosein which Y¹ denotes F and Y² denotes H or F, preferably F. Furtherpreferred compounds of the formulae I and Ia-Id are those in which X⁰denotes F or OCF₃, preferably F.

Further preferred embodiments are indicated below:

-   -   The medium additionally comprises one or more compounds of the        formulae II and/or III:    -   in which A, R⁰, X⁰, Y¹ and Y² have the meaning indicated in        formula I, and Y³ and Y⁴ denote H or F;        preferably denotes    -   The compounds of the formula II are preferably selected from the        following formulae:    -   in which R⁰ and X⁰ have the meanings indicated above. R⁰        preferably denotes alkyl having 1 to 8 C atoms and X⁰ preferably        denotes F. Particular preference is given to compounds of the        formulae IIa and IIb;    -   The compounds of the formula III are preferably selected from        the following formulae:    -   in which R⁰ and X⁰ have the meanings indicated above. R⁰        preferably denotes alkyl having 1 to 8 C atoms and X⁰ preferably        denotes F. Particular preference is given to compounds of the        formula IIIa;    -   The medium additionally comprises one or more compounds selected        from the following formulae:    -   in which R⁰, X⁰ and Y¹⁻⁴ have the meanings indicated in formula        I,    -   Z⁰ denotes —C₂H₄—, —(CH₂)₄—, —CH═CH—, —CF═CF—, —C₂F₄—, —CH₂CF₂—,        —CF₂CH₂—, —CH₂O—, —OCH₂—, —COO— or —OCF₂—, in formulae V and VI        also a single bond, in formulae V and VIII also —CF₂O—, and    -   r denotes 0 or 1;    -   The compounds of the formula IV are preferably selected from the        following formulae:    -   in which R⁰ and X⁰ have the meanings indicated above. R⁰        preferably denotes alkyl having 1 to 8 C atoms and X⁰ preferably        denotes F or OCF₃;    -   The compounds of the formula V are preferably selected from the        following formulae:    -   in which R⁰ and X⁰ have the meanings indicated above. R⁰        preferably denotes alkyl having 1 to 8 C atoms and X⁰ preferably        denotes F;    -   The compounds of the formula VII are preferably selected from        the following formulae:    -   in which R⁰ and X⁰ have the meanings indicated above. R⁰        preferably denotes alkyl having 1 to 8 C atoms and X⁰ preferably        denotes F;    -   The medium comprises one or more compounds selected from the        following formulae:

in which X⁰ has the meaning indicated in formula I and preferablydenotes F, L denotes H or F, “alkyl” denotes C¹⁻⁷-alkyl, R′ denotesC¹⁻⁷-alkyl, C¹⁻⁶-alkoxy or C²⁻⁷-alkenyl, and “alkenyl” and each,independently of one another, denote “alkenyl*” C²⁻⁷-alkenyl.

-   -   The compounds of the formulae IX-XII are preferably selected        from the following formulae:    -   in which “alkyl” and “alkyl*” each, independently of one        another, denote C₁₋₇-alkyl;    -   The medium additionally comprises one or more compounds selected        from the following formulae:    -   in which R¹ and R² each, independently of one another, denote        n-alkyl, alkoxy, oxaalkyl, fluoroalkyl or alkenyl, each having        up to 9 C atoms, and preferably each, independently of one        another, denote alkyl having 1 to 8 C atoms;    -   The medium additionally comprises one or more compounds of the        following formula:    -   in which A, B, R⁰, X⁰ and Y^(1,2) have the meanings indicated in        formula I, where A and B do not simultaneously denote        cyclohexylene;    -   The compounds of the formula XV are preferably selected from the        following formulae:    -   in which R⁰ and X⁰ have the meanings indicated above. R⁰        preferably denotes alkyl having 1 to 8 C atoms, and X⁰        preferably denotes F;    -   The medium comprises one or more compounds of the following        formula:    -   in which R¹ and R² have the meaning indicated above, and        preferably each, independently of one another, denote alkyl        having 1 to 8 C atoms, and L denotes H or F;    -   The medium additionally comprises one or more compounds selected        from the following formulae:    -   in which R^(1,2) and Y^(1,2) have the meanings indicated above;    -   The medium additionally comprises one or more compounds selected        from the following formulae:    -   in which R⁰ and X⁰ each, independently of one another, have one        of the meanings indicated above, and Y¹⁻¹² each, independently        of one another, denote H or F. X⁰ is preferably F, Cl, CF₃, OCF₃        or OCHF₂. R⁰ preferably denotes alkyl, alkoxy, oxaalkyl,        fluoroalkyl or alkenyl, each having up to 8 C atoms.        preferably    -   R⁰ is straight-chain alkyl or alkenyl having 2 to 7 C atoms;    -   X⁰ is F;    -   The medium comprises one, two or more compounds of the formula        I, in particular of the formula Ia, Ib or Ic;    -   The medium comprises 2-40% by weight, preferably 3-30% by        weight, particularly preferably 3-20% by weight, of compounds of        the formula I;    -   The medium comprises compounds selected from the formulae I, II,        III, IV, IX-XII, XIII and XIV;    -   The proportion of compounds of the formulae II, III, IV, IX-XII,        XIII and XIV in the mixture as a whole is 40 to 95% by weight;    -   The medium comprises 20-90% by weight, particularly preferably        30-80% by weight, of compounds of the formula II;    -   The medium comprises 5-60% by weight, particularly preferably        10-50% by weight, of compounds of the formula IIa;    -   The medium comprises 5-60% by weight, particularly preferably        10-50% by weight, of compounds of the formula IIb;    -   The medium comprises 2-30% by weight, particularly preferably        2-20% by weight, of compounds of the formula III;    -   The medium comprises 2-30% by weight, particularly preferably        3-20% by weight, of compounds of the formula IV;    -   The medium comprises 2-30% by weight, particularly preferably        3-20% by weight, of compounds of the formulae IX-XII;    -   The medium comprises 4-30% by weight, particularly preferably        5-25% by weight, of compounds of the formula XIV.

It has been found that even a relatively small proportion of compoundsof the formula I mixed with conventional liquid-crystal materials, butin particular with one or more compounds of the formulae II to XXIII,results in a significant increase in the light stability and in lowbirefringence values, with broad nematic phases with low smectic-nematictransition temperatures being observed at the same time, improving theshelf life. At the same time, the mixtures exhibit very low thresholdvoltages and very good values for the VHR on exposure to UV.

The term “alkyl” or “alkyl*” encompasses straight-chain and branchedalkyl groups having 1-7 carbon atoms, in particular the straight-chaingroups methyl, ethyl, propyl, butyl, pentyl, hexyl and heptyl. Groupshaving 1-6 carbon atoms are generally preferred.

The term “alkenyl” or “alkenyl*” encompasses straight-chain and branchedalkenyl groups having 2-7 carbon atoms, in particular the straight-chaingroups. Preferred alkenyl groups are C₂-C₇-1E-alkenyl, C₄-C₇-3E-alkenyl,C₅-C₇-4-alkenyl, C₆-C₇-5-alkenyl and C₇-6-alkenyl, in particularC₂-C₇-1E-alkenyl, C₄-C₇-3E-alkenyl and C₅-C₇-4-alkenyl. Examples ofparticularly preferred alkenyl groups are vinyl, 1E-propenyl,1E-butenyl, 1E-pentenyl, 1E-hexenyl, 1E-heptenyl, 3-butenyl,3E-pentenyl, 3E-hexenyl, 3E-heptenyl, 4-pentenyl, 4Z-hexenyl,4E-hexenyl, 4Z-heptenyl, 5-hexenyl, 6-heptenyl and the like. Groupshaving up to 5 carbon atoms are generally preferred.

The term “fluoroalkyl” preferably encompasses straight-chain groupshaving a terminal fluorine, i.e. fluoromethyl, 2-fluoroethyl,3-fluoropropyl, 4-fluorobutyl, 5-fluoropentyl, 6-fluorohexyl and7-fluoroheptyl. However, other positions of the fluorine orpolyfluorinated alkyl chains are not excluded.

The term “oxaalkyl” or “alkoxy” preferably encompasses straight-chainradicals of the formula C_(n)H_(2n+1)—O—(CH₂)_(m), in which n and meach, independently of one another, denote 1 to 6. m may also denote 0.Preferably, n=1 and m=1-6 or m=0 and n=1-3.

Through a suitable choice of the meanings of R⁰ and X⁰, the addressingtimes, the threshold voltage, the steepness of the transmissioncharacteristic lines, etc., can be modified in the desired manner. Forexample, 1E-alkenyl radicals, 3E-alkenyl radicals, 2E-alkenyloxyradicals and the like generally result in shorter addressing times,improved nematic tendencies and a higher ratio between the elasticconstants k₃₃ (bend) and k₁₁ (splay) compared with alkyl and alkoxyradicals. 4-Alkenyl radicals, 3-alkenyl radicals and the like generallygive lower threshold voltages and lower values of k₃₃/k₁₁ compared withalkyl and alkoxy radicals. The mixtures according to the invention aredistinguished, in particular, by high K₁ values and thus havesignificantly faster response times than the mixtures from the priorart.

The optimum mixing ratio of the compounds of the above-mentionedformulae depends substantially on the desired properties, on the choiceof the components of the above-mentioned formulae and on the choice ofany further components that may be present.

Suitable mixing ratios within the range indicated above can easily bedetermined from case to case.

The total amount of compounds of the above-mentioned formulae in themixtures according to the invention is not crucial. The mixtures cantherefore comprise one or more further components for the purposes ofoptimisation of various properties. However, the observed effect on thedesired improvement in the properties of the mixture is generallygreater, the higher the total concentration of compounds of theabove-mentioned formulae.

In a particularly preferred embodiment, the media according to theinvention comprise compounds of the formulae II to VIII (preferably II,III, IV and V, in particular IIa and IIIa), in which X⁰ denotes F, OCF₃,OCHF₂, OCH═CF₂, OCF═CF₂ or OCF₂—CF₂H. A favourable synergistic actionwith the compounds of the formula I results in particularly advantageousproperties. In particular, mixtures comprising compounds of the formulaeI, IIa and IIIa are distinguished by their low threshold voltage.

The individual compounds of the above-mentioned formulae and thesub-formulae thereof which can be used in the media according to theinvention are either known or can be prepared analogously to the knowncompounds.

The invention also relates to electro-optical displays, such as, forexample, STN or MLC displays, having two plane-parallel outer plates,which, together with a frame, form a cell, integrated non-linearelements for switching individual pixels on the outer plates, and anematic liquid-crystal mixture having positive dielectric anisotropy andhigh specific resistance located in the cell, which contain media ofthis type, and to the use of these media for electro-optical purposes.

The liquid-crystal mixtures according to the invention enable asignificant broadening of the available parameter latitude. Theachievable combinations of clearing point, viscosity at low temperature,thermal and UV stability and high optical anisotropy are far superior toprevious materials from the prior art.

The mixtures according to the invention are particularly suitable formobile applications and low-Δn TFT applications, such as, for example,mobile telephones and PDAs.

The liquid-crystal mixtures according to the invention, while retainingthe nematic phase down to −20° C. and preferably down to −30° C.,particularly preferably down to −40° C., and the clearing point ≧70° C.,preferably ≧80° C., particularly preferably ≧85° C., at the same timeallow dielectric anisotropy values Δ∈≧+10, preferably ≧+12, and a highvalue for the specific resistance to be achieved, enabling excellent MLCdisplays to be obtained. In particular, the mixtures are characterisedby low operational voltages.

The threshold voltage of the liquid-crystal mixtures according to theinvention is preferably ≦1.1 V, particularly preferably ≦1.0 V.

The birefringence Δn of the liquid-crystal mixtures according to theinvention is preferably ≧0.11, particularly preferably ≧0.09.

The rotational viscosity γ1 of the liquid-crystal mixtures according tothe invention at 20° C. is preferably ≦180 mPa·s, particularlypreferably ≦160 mPa·s.

The nematic phase range of the liquid-crystal mixtures according to theinvention preferably has a width of at least 90°, in particular at least100°. This range preferably extends at least from −40° to +80° C.

It goes without saying that, through a suitable choice of the componentsof the mixtures according to the invention, it is also possible forhigher clearing points (for example above 100° C.) to be achieved athigher threshold voltages or lower clearing points to be achieved atlower threshold voltages with retention of the other advantageousproperties. At viscosities correspondingly increased only slightly, itis likewise possible to obtain mixtures having a higher Δ∈ and thus lowthresholds. The MLC displays according to the invention preferablyoperate at the first Gooch and Tarry transmission minimum [C. H. Goochand H. A. Tarry, Electron. Lett. 10, 2-4, 1974; C. H. Gooch and H. A.Tarry, Appl. Phys., Vol. 8, 1575-1584, 1975], where, besidesparticularly favourable electro-optical properties, such as, forexample, high steepness of the characteristic line and low angledependence of the contrast (German patent 30 22 818), lower dielectricanisotropy is sufficient at the same threshold voltage as in ananalogous display at the second minimum. This enables significantlyhigher specific resistance values to be achieved using the mixturesaccording to the invention at the first minimum than in the case ofmixtures comprising cyano compounds. Through a suitable choice of theindividual components and their proportions by weight, the personskilled in the art is able to set the birefringence necessary for apre-specified layer thickness of the MLC display using simple routinemethods.

Measurements of the voltage holding ratio (HR) [S. Matsumoto et al.,Liquid Crystals 5, 1320 (1989); K. Niwa et al., Proc. SID Conference,San Francisco, June 1984, p. 304 (1984); G. Weber et al., LiquidCrystals 5, 1381 (1989)] have shown that mixtures according to theinvention comprising compounds of the formula I exhibit a significantlysmaller decrease in the HR on UV exposure than analogous mixturescomprising cyano-phenylcyclohexanes of the formula

or esters of the formula

instead of the compounds of the formula I.

The light stability and UV stability of the mixtures according to theinvention are considerably better, i.e. they exhibit a significantlysmaller decrease in the HR on exposure to light or UV. Even lowconcentrations of the compounds (<10% by weight) of the formula I in themixtures increase the HR by 6% or more compared with mixtures from theprior art.

The construction of the MLC display according to the invention frompolarisers, electrode base plates and surface-treated electrodescorresponds to the usual design for displays of this type. The termusual design is broadly drawn here and also encompasses all derivativesand modifications of the MLC display, in particular including matrixdisplay elements based on poly-Si TFTs or MIM.

A significant difference between the displays according to the inventionand the hitherto conventional displays based on the twisted nematic cellconsists, however, in the choice of the liquid-crystal parameters of theliquid-crystal layer.

The liquid-crystal mixtures which can be used in accordance with theinvention are prepared in a manner conventional per se, for example bymixing one or more compounds of the formula I with one or more compoundsof the formulae II-XXIII or with further liquid-crystalline compoundsand/or additives. In general, the desired amount of the components usedin lesser amount is dissolved in the components making up the principalconstituent, advantageously at elevated temperature. It is also possibleto mix solutions of the components in an organic solvent, for example inacetone, chloroform or methanol, and to remove the solvent again, forexample by distillation, after thorough mixing.

The dielectrics may also comprise further additives known to the personskilled in the art and described in the literature, such as, forexample, UV stabilisers, such as Tinuvin® from Ciba, antioxidants,free-radical scavengers, nanoparticles, etc. For example, 0-15% ofpleochroic dyes or chiral dopants can be added. Suitable stabilisers anddopants are mentioned below in Tables C and D.

In the present application and in the examples below, the structures ofthe liquid-crystal compounds are indicated by means of acronyms, thetrans-formation into chemical formulae taking place in accordance withTables A and B below. All radicals C_(n)H_(2n+1) and C_(m)H_(2m+1) arestraight-chain alkyl radicals having n and m C atoms respectively; n andm are integers and preferably denote 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10,11 or 12. The coding in Table B is self-evident. In Table A, only theacronym for the parent structure is indicated. In individual cases, theacronym for the parent structure is followed, separated by a dash, by acode for the substituents R¹*, R²*, L¹* and L²*: Code for R¹*, R²*, L¹*,L²*, L³* R¹* R²* L¹* L²* nm C_(n)H_(2n+1) C_(m)H_(2m+1) H H nOmC_(n)H_(2n+1) OC_(m)H_(2m+1) H H nO•m OC_(n)H_(2n+1) C_(m)H_(2m+1) H H nC_(n)H_(2n+1) CN H H nN•F C_(n)H_(2n+1) CN F H nN•F•F C_(n)H_(2n+1) CN FF nF C_(n)H_(2n+1) F H H nCl C_(n)H_(2n+1) Cl H H nOF OC_(n)H_(2n+1) F HH nF•F C_(n)H_(2n+1) F F H nF•F•F C_(n)H_(2n+1) F F F nOCF₃C_(n)H_(2n+1) OCF₃ H H nOCF₃•F C_(n)H_(2n+1) OCF₃ F H n-Vm C_(n)H_(2n+1)—CH═CH—C_(m)H_(2m+1) H H nV—Vm C_(n)H_(2n+1)—CH═CH— —CH═CH—C_(m)H_(2m+1)H H

Preferred mixture components are shown in Tables A and B. TABLE A

CCH

BCH

ECCP

CBC

CCP

PCH

TABLE B

CCQU-n-F

CCQG-n-F

ACQU-n-F

PUQU-n-F

CCZU-n-F

CDU-n-F

CCP-nV-m

CC-nV-Vm

CCG-V-F

CC-n-V

CCOC-n-m

CH-n-m

CAUQU-n-F

DAUQU-n-F

ACUQU-n-F

ADUQU-n-F

Particular preference is given to liquid-crystalline mixtures which,besides the compounds of the formula I, comprise at least one, two,three, four or more compounds from Table A or B. TABLE C Table Cindicates possible dopants which are generally added to the mix- turesaccording to the invention. The mixtures preferably comprise 0-10% byweight, in particular 0.01-5% by weight and particularly preferably0.01-3% by weight of dopants.

C 15

CB 15

CM 21

R/S-811

CM 44

CM 45

CM 47

CN

R/S-2011

R/S-3011

R/S-4011

R/S-5011

R/S-1011

TABLE D Stabilisers which can be added, for example, to the mixturesaccording to the invention in amounts of 0-10% by weight are mentionedbelow.

The following examples are intended to explain the invention withoutlimiting it.

Above and below, percentage data denote percent by weight. Alltemperatures are indicated in degrees Celsius. m.p. denotes meltingpoint, cl.p.=clearing point. Furthermore, C=crystalline state, N=nematicphase, S=smectic phase and I=isotropic phase. The data between thesesymbols represent the transition temperatures. Furthermore,

-   -   Δn denotes the optical anisotropy at 589 nm and 20° C.,    -   γ₁ denotes the rotational viscosity (mPa·s) at 20° C.,    -   V₁₀ denotes the voltage (V) for a 10% transmission (viewing        angle perpendicular to the plate surface), (threshold voltage),    -   V₉₀ denotes the voltage (V) for a 90% transmission (viewing        angle perpendicular to the plate surface),    -   Δ∈ denotes the dielectric anisotropy at 20° C. and 1 kHz        (Δ∈=∈_(∥)−∈_(⊥), where ∈₈₁ denotes the dielectric constant        parallel to the longitudinal axes of the molecules and ∈_(⊥)        denotes the dielectric constant perpendicular thereto).

The electro-optical data are measured in a TN cell at the 1st minimum(i.e. at a d·Δn value of 0.5 μm) at 20° C., unless expressly indicatedotherwise. The optical data are measured at 20° C., unless expresslyindicated otherwise. All physical properties are determined inaccordance with “Merck Liquid Crystals, Physical Properties of LiquidCrystals”, status November 1997, Merck KGaA, Germany, and apply for atemperature of 20° C., unless explicitly indicated otherwise.

COMPARATIVE EXAMPLE 1

CCP-3F•F•F 12.00% Clearing point [° C.]: 90.0 CCQU-2-F 8.00% Δn [589 nm,20° C.]: 0.0710 CCQU-3-F 8.00% Δε [kHz, 20° C.]: +10.5 CCQU-5-F 8.00% γ₁[mPa · s, 20° C.]: 157 PUQU-2-F 10.00% V₁₀ [V]: 1.29 CCOC-4-3 4.00% V₉₀[V]: 1.97 CCOC-3-3 4.00% CCOC-3-5 4.00% CH-43 3.00% CH-45 2.00% CCH-30112.00% CCH-303 3.00% CCZU-2-F 4.00% CCZU-3-F 14.00% CCZU-5-F 4.00%

EXAMPLE 1

CCQG-3-F 5.00% Clearing point [° C.]: 89.5 CCQU-2-F 11.00% Δn [589 nm,20° C.]: 0.0710 CCQU-3-F 14.00% V₁₀ [V]: 1.00 CCQU-5-F 12.00% V₉₀ [V]:1.55 PUQU-3-F 5.00% CCOC-4-3 4.00% CCOC-3-3 3.00% CCOC-3-5 2.00%ACQU-2-F 12.00% ACQU-3-F 12.00% ACQU-4-F 13.00% CAUQU-3-F 7.00%

The mixture has a significantly lower threshold voltage compared withthe mixture from Comparative Example 1 with the same birefringence andvirtually the same clearing point.

EXAMPLE 2

CCP-5F•F•F 5.00% Clearing point [° C.]: 85.5 CCQU-2-F 11.00% Δn [589 nm,20° C.]: 0.0792 CCQU-3-F 13.00% V₁₀ [V]: 0.96 CCQU-5-F 11.00% V₉₀ [V]:1.51 PUQU-3-F 8.00% ACQU-2-F 11.00% ACQU-3-F 12.00% ACQU-4-F 11.00%CCP—V-1 10.00% CAUQU-3-F 8.00%

The mixture has a significantly lower threshold voltage compared withthe mixture from Comparative Example 1 with virtually the samebirefringence and a somewhat lower clearing point.

The preceding examples can be repeated with similar success bysubstituting the generically or specifically described reactants and/oroperating conditions of this invention for those used in the precedingexamples.

The entire disclosures of all applications, patents and publications,cited herein and of corresponding German application No. 10 2006 046906.2, filed Oct. 4, 2006 are incorporated by reference herein.

From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention and, withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions.

1. A liquid-crystalline medium, comprising one or more compounds of formula I

in which R⁰ denotes a halogenated or unsubstituted alkyl or alkoxy radical having 1 to 15 C atoms, in which one or more CH₂ groups are optionally each, independently of one another, replaced by —C≡C—,

in such a way that O atoms are not linked directly to one another, X⁰ denotes F, Cl, CN, SF₅, SCN, NCS, a halogenated alkyl radical, a halogenated alkenyl radical, a halogenated alkoxy radical or a halogenated alkenyloxy radical having up to 6 C atoms, and Y¹ and Y² each, independently of one another, denote H or F, and

each, independently of one another, denote

where at least one of

denotes


2. A liquid-crystalline medium according to claim 1, comprising one or more compounds of the following formulae:

in which R⁰ and X⁰ have the meanings indicated for formula I.
 3. A liquid-crystalline medium according to claim 1, further comprising one or more compounds of the following formulae:

in which A, R⁰, X⁰, Y¹ and Y² have the meanings indicated for formula I, and Y³ and Y⁴ each, independently of one another, denote H or F.
 4. A liquid-crystalline medium according to claim 3, comprising one or more compounds of the following formulae:

in which R⁰ and X⁰ have the meanings indicated for formula I.
 5. A liquid-crystalline medium according to claim 1, further comprising one or more compounds of the following formulae:

in which R⁰, X⁰, Y¹ and Y² have the meanings indicated for formula I, Y³ and Y⁴ each, independently of one another, denote H or F, Z⁰ denotes —C₂H₄—, —(CH₂)₄—, —CH═CH—, —CF═CF—, —C₂F₄—, —CH₂CF₂—, —CF₂CH₂—, —CH₂O—, —OCH₂—, —COO— or —OCF₂—, in formulae V and VI also a single bond, in formulae V and VIII also —CF₂O—, and r denotes 0 or
 1. 6. A liquid-crystalline medium according to claim 5, comprising one or more compounds of the following formulae:

in which R⁰ and X⁰ have the meanings indicated for formula I.
 7. A liquid-crystalline medium according to claim 1, comprising one or more compounds of the following formulae:

in which X⁰ has the meaning indicated for formula I, L denotes H or F, “alkyl” denotes C¹⁻⁷-alkyl, R′ denotes C¹⁻⁷-alkyl, C¹⁻⁶-alkoxy or C²⁻⁷-alkenyl, and “alkenyl” and each, independently of one another, denote “alkenyl*” C²⁻⁷-alkenyl.


8. A liquid-crystalline medium according to claim 1, comprising one or more compounds of the following formulae:

in which R¹ and R² each, independently of one another, denote n-alkyl, alkoxy, oxaalkyl, fluoroalkyl or alkenyl, each having up to 9 C atoms.
 9. A liquid-crystalline medium according to claim 3, comprising 2-40% by weight one or more compounds of formula I, 20-90% by weight one or more compounds of formula II, 2-30% by weight one or more compounds of formula III, optionally 2-30% by weight one or more compounds of formula IV,

in which R⁰, X⁰, Y¹ and Y² have the meanings indicated for formula I, optionally 2-30% by weight one or more compounds of formulae IX-XII,

in which X⁰ has the meaning indicated for formula I, L denotes H or F, “alkyl” denotes C¹⁻⁷-alkyl, R′ denotes C¹⁻⁷-alkyl, C¹⁻⁶-alkoxy or C²⁻⁷-alkenyl, and “alkenyl” and each, independently of one another, denote “alkenyl*” C²⁻⁷-alkenyl

and optionally 4-30% by weight one or more compounds of formula XIV

in which R¹ and R² each, independently of one another, denote n-alkyl, alkoxy, oxaalkyl, fluoroalkyl or alkenyl, each having up to 9 C atoms.
 10. An electro-optical apparatus, comprising a liquid-crystalline medium according to claim
 1. 11. An electro-optical liquid-crystalline display, containing a liquid-crystalline medium according to claim
 1. 12. A process for preparing a liquid-crystalline medium according to claim 1, comprising mixing together one or more compounds of formula I with further liquid-crystalline compounds and/or additives. 